Study of fluorination of CdTe surfaces

Thin Solid Films, 198 (1991 ) 347 355

347

PREPARATION AND CHARACTERIZATION

STUDY OF FLUORINATION OF CdTe SURFACES KOICHI SUGIYAMA, KOICHI MORI AND HIDETO MIYAKE

Department of Electrical Engineering, Mie University, Kamihama-cho, Tsu-shi, Mie 514 (Japan) (Received August 8, 1990; accepted October l, 1990)

In this paper we deal with fluorination of CdTe(100) single crystals in a 2~o fluorine-98)/o nitrogen atmosphere for different temperatures and times. The fluorination process has been investigated by the use of microscopy, X-ray diffraction, energy-dispersive X-ray microanalysis, X-ray photoelectron spectroscopy and Auger electron spectroscopy measurements. Three temperature regions are found to be distinguished for the fluorination process. The fluorinated layer is mainly composed of CdF2 crystals, but an intermediate layer is shown to exist beneath the fluoride layer except for fluorination at low temperature. The formation mechanisms of the fluoride and intermediate layers are discussed.

1. INTRODUCTION The II-VI compound CdTe has recently attracted much research interest, since it has promising applications for photovoltaic devices. Heterostructure p-n junctions such as n-CdS/p-CdTe and n-(indium tin oxide)/p-CdTe 1, and metal/ insulator/semiconductor structures such as metal/CdTeO3/n-CdTe2 and metal/Langmuir film/n-CdTe 3 have been investigated for the improvement of device performance. As possible alternative combinations of materials for these structures, nCdF2/p-CdTe and metal/i-CdF2/p-CdTe systems deserve much consideration, since CdF 2 is a compound with a band gap of 7.8 eV4, and is an insulator without doping, and becomes an n-type semiconductor with an electron concentration of up to 7 × 1018 cm-3 by doping with trivalent impurities and subsequent annealing in saturated cadmium vapour 5'6. The CdF2 layers are expected to be prepared by fluorination of the surfaces of CdTe single crystals by heating in a fluorine atmosphere. The fluorination of surfaces of HgxCd 1_xTe alloys has been investigated using anodic oxidation 7, but, to our knowledge, fluorination of CdTe and HgxCd~_,Te by heating in an ambient containing fluorine gas has not been reported. Since the CdF2 layers prepared on CdTe crystals at rather high temperatures in the gaseous phase might be quite thick, there is also the possibility that they might be used as a material for light-emitting devices 8. 0040-6090/91/$3.50

© ElsevierSequoia/Printedin The Netherlands

348

K. S U G I Y A M A , K. MORI, H. MIYAKE

In this work we investigated the fluorination of CdTe single crystals by heating in an ambient containing fluorine gas. Detailed information and a better understanding of the fluorination process will be useful for device applications of the CdF2 films. 2. E X P E R I M E N T A L P R O C E D U R E S

(100)-oriented n- and p-type CdTe single-crystal wafers with dimensions of about 5 m m × 5 mm x 1 m m were used in this experiment. After conventional surface treatment, they were placed on a nickel boat in a Monel tube of 11 m m inner diameter, through which a 20,, fluorine-98'!,, nitrogen mixture was passed at a rate of 100cm 3 min 1. Fluorination was performed by heating the samples at the desired temperatures ranging from 100 to 450 ~'C for 1-100 min. After the fluorination, the cleaved cross-section was examined in optical and scanning electron microscopes. The distribution of elemental components through the depth was measured on the cleaved plane by an energy-dispersive X-ray (EDX) microanalyser. Since elements with atomic numbers lower than 10 cannot be detected by this instrument, the atom fraction of fluorine in the samples could not be determined. For some of the samples with a thin fluorinated layer, the depth distribution of constituent elements including fluorine was obtained by using Auger electron spectroscopy (AES) combined with ion sputter etching. The structure of the surfaces of the fluorinated layers was also studied by X-ray diffraction (XD) and Xray photoelectron spectroscopy (X PS). 3.

RESULTS A N D D I S C U S S I O N

Examples of photomicrographs of surfaces and cleaved cross-sections of fluorinated CdTe samples are shown in Fig. 1. Two layers are seen for samples treated at temperatures ranging from 130 to 450 :C as shown in Figs. l(b') and l(c'), whereas only a single layer is observed in samples fluorinated at t00°C (Fig. l(a')). Thin layers (about 0.4 I,tm) fluorinated at 100 ~C are smooth and without cracks, but thick fluorinated layers are generally cracked as shown in Figs. l(a)-l(c). This seems to be due to the large lattice mismatch or large difference of thermal expansion coefficients between CdTe 9'1° and CdF2 11'12. The samples fluorinated at 400-450~C have many round spots on the surface in addition to the cracks (Fig. l(c)). Figure 2 shows scanning electron microscopy (SEM) micrographs of the cross sections of the samples prepared at 130-450 "C. The presence of two layers is also indicated in the figures. The surface layers fluorinated at 100 ~"C and the upper layers of the samples fluorinated at 130-450 °C are labelled layer 1, since all of them seem to be mainly composed of C d F 2, as described in the following paragraphs. The thickness of layer 1 was ascertained to be proportional to the square root of the fluorination time of 1-100 min, suggesting that diffusion is the rate-determining process. The thickness d of layer 1 for a fluorination time of 10min is plotted as a function of reciprocal temperature in Fig. 3. The result indicates that three temperature regions can be distinguished concerning the fluorination process: (1) a low temperature region

FLUORINATION OF

CdTe SURFACES

349

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around i00 °C, (2) an intermediate temperature region of 130-370 °C and (3) a high temperature region of 4(K1--450 °C. The thickness of layer 1 for region (2) is markedly larger than those for regions (1) and (3), and its temperature dependence can be represented as d oc exp(- AE/k T), where AE = 0.14 eV. The intermediate layers of samples fluorinated in regions (2) and (3) are labelled layer 2 and layer 2' respectively.

350

K. SUGIYAMA, K. MORt, H. MIYAKE

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FLUORINATION OF C d T e SURFACES

351

Since the tellurium concentrations in layer 1 of the samples fluorinated at the intermediate and high temperatures are nearly zero, layer 1 is considered to be predominantly composed of CdF2. Layer 2 in samples prepared by the intermediate-temperature fluorination has a Te:(Cd+Te) ratio of 0.5; hence, the layer is a part of the CdTe substrates, which might be heavily doped with fluorine in-diffused through layer 1. The fluorination in the intermediate temperature region may be explained as follows. Fluorine atoms from the vapour phase diffuse into the CdTe (through layer 1) and then they combine with cadmium atoms to form a layer of CdF 2 crystals by replacing tellurium atoms, because the free energy of formation of CdF 2 is much larger than that of CdTe 13. The resulting excess tellurium atoms occupy interstitial sites in the CdF2 crystals. Hence, they are mobile and diffuse out through layer 1 to the surface and form compounds with fluorine, which evaporate easily into the vapour phase because of their low boiling or sublimation temperatures 14. The fluorination process is suggested to be diffusion limited. However, it was not possible to decide which mechanism is the rate-limiting process, fluorine in-diffusion or tellurium out-diffusion, because the fluorine profile in the samples could not be obtained with the instrument used in this work. Layer 2' in samples prepared at the highest temperatures has a composition different from that of layer 2. Significant build-up of tellurium takes place in the layer as shown in Fig. 4(b). Since stable solid or liquid tellurium fluorides are not known at 40(0450 °C 14, layer 2' is thought to be composed of tellurium polycrystals. (With fluorination at 450 °C, the possibility of tellurium's being liquid cannot be ruled out, because its melting point is 449.8 °C.) The accumulation of tellurium atoms in layer 2' is interpreted in terms of the following model. At high temperatures (400-450 °C), diffusion of cadmium, which is presumably enhanced by the electric field 1s induced by Cd 2 + and excess F - , may become much faster than that of tellurium and fluorine because of the small radius of the cadmium ion~6; hence, the cadmium ions will migrate to the surface and combine with fluorine atoms. The remaining tellurium atoms precipitate and form crystals, and the atoms in these crystals are rather stable and rarely out-diffuse through layer 1. The Cd 2 ÷ ion may be produced in CdTe thermally or by radiation from the heaters of the electric furnace. In order to verify the above-mentioned models, the change in sample weights caused by the fluorination for 10min was measured. The weight decreases by 3%8% after fluorination at the intermediate temperatures, which supports the suggestion that the change is mainly due to out-diffusion of tellurium atoms replaced by fluorine. On the contrary, the weight increases by about 1% for the high temperature fluorination, which supports the model where tellurium out-diffusion is negligible and fluorine in-diffusion is dominant. For the samples fluorinated in the low temperature region, EDX measurements were performed on the surface of layer 1 with a thickness of 2.8 Ixm.The Te:(Cd + Te) ratio is in the range 0.01-0.06, and this indicates that layer 1 is mainly composed of CdF2 as in the cases of the intermediate and high temperature fluorination. For samples with a thin layer (about 0.4 ~tm) fluorinated at 100 °C for 3 min, the depth profiles of composition were obtained using the AES technique. An example is shown in Fig. 5. Tellurium out-diffusion and cadmium accumulation in the surface

352

K. SUGIYAMA, K. MORI, H. MIYAKE

layer were also observed, but the concentration of fluorine in the surface layer is considerably lower than that of stoichiometric CdF 2. As fluorine atoms are mobile during the sputter etching, this might have an influence on the experimental result. Typical examples of XD patterns from the surfaces of fluorinated samples are shown in Fig. 6. Only the XD lines from CdF 2 crystals were observed, which confirms the speculation that layer 1 is mainly composed of CdF 2 crystals. The CdF 2 crystals in layer 1, which were prepared at 100 °C for 10-60min (thickness, 2.8-14 lam), are [100] oriented as shown in Fig. 6(a). The layers seem to be grown in the same direction as that of the CdTe substrates, despite the large lattice mismatch between CdF2 and CdTe (20.3~o). This epitaxial relation might be associated with the property that a (6 × 6) CdF 2 superlattice unit cell is nearly exactly commensurate with a (5 × 5) CdTe cell, as in the case of the CdTe/GaAs structure 17.

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When the fluorination was carried out at the intermediate temperatures, the prepared layers (thickness, 12-200 ~tm) have several XD peaks as shown in Fig. 6(b). This result means that layer 1 consists of grains having different orientations. The presence of grains seems to be closely related to the columnar structures in layers 1 and 2 shown in Fig. 2(a). The high growth rate of layer 1 in this region might be related to the diffusion of tellurium and fluorine atoms along the grain boundaries. Layer 1 for the high temperature fluorination has two XD peaks as shown in Fig. 6(c). Since layer 2' exists between layer 1 and the CdTe substrate, the relation of crystal orientation between layer 1 and the substrate may be weakened. The low growth rate of layer 1 in this temperature region may also be due to the presence of layer 2' through which cadmium ions should inevitably have to diffuse. Examples of XPS spectra for the surfaces of fluorinated layers are shown in

FLUORINATION OF C d T e SURFACES

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Temperature region (°C)

CdTe

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• The value relative to the standard state. b Fluorination time. c Fluorination temperature, 200 °C. d After ion sputter etching (about 100 A), tellurium peaks disappeared.

2.3 -2.3 4.8

354

K. SUGIYAMA, K. MORI, H. M1YAKE

Fig. 7. Tellurium peaks are not observed for samples fluorinated at the low and intermediate temperatures as shown in Fig. 7(a), which also confirms that layer 1 is composed of CdF2 crystals. The chemical shifts of the elements are listed in Table I, where other properties are also summarized. The comparison of the chemical shifts for layer 1 with those for commercially available CdF 2 powders also confirmed that layer 1 is CdF 2. The XPS spectrum of a sample fluorinated at the highest temperature has tellurium peaks as shown in Fig. 7(b), and they exhibit positive chemical shifts (Table I). The tellurium peaks may be related to the spots observed on the surface of the sample (Fig. l(c)). Since the vapour pressure of tellurium at 400-450 °C is relatively high, the vapour from layer 2' may pass through pinholes in layer 1 and may condense at the peripheries of the pinholes on the surface of the layer as round spots during or after the fluorination process; they may then be oxidized by a more electronegative element such as oxygen remaining as an impurity in the nitrogen atmosphere t8. In fact, the intensity of the O ls peak is rather high in Fig. 7(b) in comparison with that in Fig. 7(a). When a thin surface layer of about 100/~ in thickness was removed by sputtering from layer 1, the tellurium peaks disappeared and the intensity of the O ls peak was diminished as shown in Fig. 7(b'). This result also confirms the suggestion that tellurium oxide is deposited on the surface. 4. CONCLUSION The fluorination of the surfaces of (100)-oriented CdTe crystals has been investigated. The fluorination was carried out in an atmosphere of2~o fluorine-98Yo nitrogen mixture at 100-450°C for 1-100min. Three temperature regions can be distinguished: (1) with fluorination at 100°C, a CdF 2 layer is prepared and is oriented along the [100] direction; (2) with fluorination at 130-370°C, the CdF 2 layer is composed of grains with several different orientations, and an intermediate layer consisting of modified CdTe is produced in addition to the CdF2 overlayer; (3) at high fluorination temperatures of 400-450 °C, a tellurium-rich intermediate layer has been grown. In temperature regions (1) and (2), fluorine in-diffusion and tellurium out-diffusion through the fluorinated layer and the subsequent evaporation of tellurium fluorides may be the dominant mechanisms, whereas in temperature region (3) cadmium ions migrate to the surface, leaving tellurium atoms behind. ACKNOWLEDGMENTS

The authors are grateful to Dr. K. Sugii of N T T Basic Research Laboratories for the AES measurements. Thanks are also due to Dr. N. Kitamura of Suzuka College of Technology, and to Professor O. Yamamoto and Professor Y. Takeda for permission to use the XPS and XD facilities respectively. REFERENCES 1 J . G . Werther, A. L. Fahrenbruch, R. H. Bube and J. C. Zesch, J. Appl. Phys., 54 (1983) 2750.

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2 F.F. Wang, A. L. Fahrenbruch and R. H. Bube, J. Appl. Phys., 65 (1989) 3552. 3 G.G. Roberts, M. C. Petty and I. M. Dharmadasa, lEE Proc., 128 (Pt. 1) (1981) 197. 4 B.A. Orlowski and P. Plenkiewicz, Phys. Status Solidi B, 126 (1984) 285. 5 J.D. Kingsley and J. S. Prener, Phys. Rev. Lett., 8 (1962) 315. 6 R.P. Khosla, Phys. Rev., 183 (1969) 695. 7 E. Weiss and N. Mainzer, J. Vac. Sei. Technol. A, 6 (1988) 2765. 8 T. Langer, B. Krukowska-Fulde and J. M. Langer, Appl. Phys. Lett., 34 (1979) 216. 9 R.C. Weast (ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 70th edn., 1989, p. E- 106. 10 J.S. Browder and S. S. Ballard, Appl. Opt., 8 (1969) 793. 11 H.M. Haendler and W. J. Bernard, J. Am. Chem. Soc., 73 (1951) 5218. 12 B. Krukowska-Fulde and T. Niemyski, J. Cryst. Growth, 1 (1967) 183. 13 C. Canali, G. Ottaviani, R. O. Bell and F. V. Wald, J. Phys. Chem. Solids, 35 (1974) 1405. 14 W.C. Cooper (ed.), Tellurium, Van Nostrand Reinhold, New York, 1971, p. 135. 15 C. Vfizquez-L6pez, F. Sfinchez-Sinencio, J. S. Helman, J. L. Pefia, A. Lastras-Martinez, P. M. Raccah and R. Triboulet, J. Appl. Phys., 50 (1979) 5391. 16 C. Kittel, Introduction to SolidState Physics, Wiley, New York, 6th edn., 1986, p. 76. 17 N. Ohtsuka, L. A. Kolodziejski, R. L. Gunshor, S. Datta, R. N. Bicknell and J. F. Schetzina, Appl. Phys. Lett., 46 (1985) 860. 18 M.K.Bahl, R. L. Watson and K. J. Irgolic, J. Chem. Phys., 66 (1977) 5526.